Transmission and Reception Methods for a Binary Signal on a Serial Link

ABSTRACT

A method can be used for transmission of at least one packet of at least one bit over a serial link capable of taking two different states respectively associated with the two possible logical values of the at least one transmitted bit. Starting from a transmission start time of the at least one bit and up to the expiration of a first portion of a bit time associated with the at least one bit, the link is placed in one of its states depending on the logical value of the at least one bit. Upon the expiration of the first portion of this bit time, a first additional transition is generated over the link so as to place the link in its other state up to the expiration of the bit time.

This application claims priority to French Application No. 1461892,filed on Dec. 4, 2014, which application is hereby incorporated hereinby reference.

TECHNICAL FIELD

Embodiments of the present invention relate to methods for transmissionand for receiving a binary signal over a serial link, in particular, forthe detection of the speed of transmission or baud rate, andcorresponding devices.

BACKGROUND

Each transmitter/receiver in communication must initially define thespeed of transmission in order to begin the communication. When thespeed of transmission needs to be modified, the communication must beinterrupted, protocol messages must be exchanged, and there are oftennumerous timing constraints to be complied with before interrupting andrestarting a communication, in particular when the packets of bits beingexchanged are asynchronous, in other words timed by non-synchronizedtransmission and reception clocks.

The current mechanisms are based on synchronization binary patternswhich must firstly be transmitted between the two devices.

The device receiving this synchronization pattern can for examplecompare it with reference patterns previously stored in a table(“look-up table”) so as to deduce from this the real speed oftransmission (baud rate).

Aside from the fact that the use of such synchronization patternsrequires proprietary protocols, which represents an impediment to theinteroperability between devices, this only allows the detection of thespeed of transmission for the start of the communication, andfurthermore from amongst a predefined set of transmission speeds, andassumes that this speed of transmission will remain constant for theentire duration of the communication.

Moreover, such a solution is completely ineffective when the speed oftransmission has to be changed in the course of the communication.

SUMMARY

Various embodiments of the invention relate to the communication ofbinary information over a serial link capable of taking two differentstates respectively associated with the two possible logical values ofthe binary information being communicated, notably the communication ofbinary information between two universal asynchronousreceiver/transmitters known by those skilled in the art under theacronym UART, in particular, with a view to the auto-detection of thespeed of transmission at the start of the transmission and/or to apotential modification of this speed of transmission in the course of acommunication.

In accordance with a preferred embodiment of the present invention, amethod for the speed of transmission or the “baud rate” is generallyexpressed in bauds which represent the number of symbols transmitted persecond (equivalent to the number of bits transmitted per second if eachsymbol corresponds to a single bit). This data rate allows the value ofthe bit time, in other words the duration of the time segment used forthe transmission of the bit, to be defined.

According to one embodiment and its implementation, methods fortransmission and reception are provided that allow, in the receiver, anauto-detection of the speed of transmission at the start of thecommunication together with an auto-detection of any potentialmodification of this speed of transmission in the course of thecommunication.

According to one embodiment and its implementation, in transmission, theidea is to use the final part of the bit time of a bit for generating atransition (or edge) over the link at the end of a portion of this bittime. This transition will be a kind of pre-warning of the imminentexpiration of the bit time. Then, in reception, the detection of thistransition will allow the receiver, in view of its knowledge of thevalue of the portion of bit time, to deduce from this the remainingperiod of time for receiving the current bit and, consequently, thespeed of transmission.

Thus, according to one aspect, a method of transmission is provided forat least one packet of at least one bit over a serial link capable oftaking two different states respectively associated with the twopossible logical values of the at least one transmitted bit; the methodcomprises a first transmission processing comprising, starting from atransmission start time of the at least one bit and up to the expirationof a first portion of a bit time associated with the at least one bit,the placing of the link in one of its states depending on the logicalvalue of the at least one bit and, upon the expiration of the firstportion of this bit time, a generation of a first additional transitionover the link so as to place the link in its other state up to theexpiration of the bit time.

So as to avoid errors leading to the detection of false bits, it ispreferable for the first additional transition to be sufficiently farfrom the middle of the bit time and from the final end of the bit time.Those skilled in the art will know how to adjust the value of the firstportion of bit time as a function notably of the value of the clocksignal. However, by way of example, the first portion of the bit timemay be in the range between 60% and 90% of the bit time.

In general, the packet comprises several bits. In this case, accordingto one embodiment, the first transmission processing is implemented atleast for the transmission of the first bit and the transmission of thesecond bit begins at the expiration of the bit time associated with thefirst bit.

This said, several variants are possible.

Thus, when the bit time associated with the first bit of the packet islonger than the bit time associated with the other bits of the packetand the bit time associated with the other bits of the packet may bedetermined, by a known relationship, from the bit time associated withthe first bit of the packet, the first transmission processing may beimplemented only for the transmission of the first bit.

Indeed, when this first bit, for example a bit referred to as “startbit”, is sufficiently long, a single transition may be sufficient fordetecting, in reception, the speed of transmission and consequently forcorrectly reading the following bits of the packet.

As a variant, when the bit time associated with all the bits of thepacket are identical, or different for some of them but determinable bya known relationship, it is possible of generate additional transitionsfor the first bit of the packet and for certain other bits of thepacket, for example the second and the fourth, which may then besufficient for detecting, in reception, the speed of transmission withsufficient precision, and consequently for correctly reading the bits ofthe packet.

Thus, according to one embodiment, the first transmission processing isimplemented for the transmission of the first bit and the transmissionof at least one other bit of the packet comprises a second transmissionprocessing comprising starting from the transmission start time of theat least one other bit and up to the expiration of a second periodcounted starting from the transmission start time of the first bit andequal to the sum of all the bit times associated with the bit(s)preceding the at least one other bit and of a second portion of the bittime associated with the at least one other bit, the placing of the linkin one of its states depending on the logical value of the at least oneother bit, and, upon the expiration of the second period, a generationof a second additional transition over the link so as to place the linkin its other state up to the expiration of the bit time associated withthe at least one other bit.

As a variant, when the bit time is not identical for all the packets, inother words for example when the speed of transmission changes in thecourse of the communication, the first transmission processing can beimplemented for the transmission of each bit of the packet and thetransmission of the current bit, from the second bit up to the last,then begins at the expiration of the bit time associated with thepreceding bit.

According to another aspect, a method is provided for reception of atleast one packet of at least one bit, carried over a serial link capableof taking two different states respectively associated with the twopossible logical values of the at least one received bit, comprising afirst reception processing comprising a detection of the occurrence of afirst transition between the two states of the link after the receptionstart time of the at least one bit and a determination of a first periodseparating the reception start time and the time of occurrence of thefirst transition, the reception end time of the at least one bit beingseparated from the time of occurrence of the first transition by a firstadditional period equal to a first predetermined portion of the firstperiod.

In the same way as the first portion of bit time was for example in therange between 60% and 90% of the bit time in transmission, during thereception, the first predetermined portion can be in the range between10% and 70% of the first period (which corresponds to a percentage ofthe bit time in the range between 10% and 40%).

According to one embodiment in which the received packet comprisesseveral bits, the first reception processing is implemented at least forthe reception of the first bit, and the reception of the second bitbegins at the expiration of the first additional period associated withthe first bit.

According to one embodiment in which, when the bit time associated withthe first bit of the received packet is longer than the bit timeassociated with the other bits of the received packet and the bit timeassociated with the other bits of the received packet may be determined,by a known relationship, from the bit time associated with the first bitof the received packet, the first reception processing may beimplemented only for the reception of the first bit.

According to one embodiment in which all the bits of the received packethave the same bit time or have different bit times for some of them butdeterminable by a known relationship, the first reception processing isimplemented for the reception of the first bit and the reception of atleast one other bit of the packet comprises a second receptionprocessing comprising a detection of the occurrence of a secondtransition between the two states of the link after the reception starttime of the at least one other bit and a determination of a secondperiod separating the reception start time of the first bit and the timeof occurrence of the second transition. The reception end time of the atleast one other bit is separated from the time of occurrence of thesecond transition by a second additional period equal to a secondpredetermined portion of the second period.

The first portion and the second portion may be identical.

According to one embodiment notably allowing a detection of a change ofspeed of transmission, potentially at each bit, the first receptionprocessing is implemented for the reception of each bit of the receivedpacket and the reception of the current bit, from the second bit up tothe last, begins at the expiration of the additional period associatedwith the preceding bit.

According to another aspect, a processing unit is provided that isconfigured for transmitting at least one packet of at least one bit overa serial link capable of taking two different states respectivelyassociated with the two possible logical values of the at least onetransmitted bit.

According to one general feature of this other aspect, the processingunit comprises transmission processing unit configured for implementingthe transmission method such as defined hereinbefore.

According to another aspect, a processing unit is provided that isconfigured for receiving at least one packet of at least one bit carriedover a serial link capable of taking two different states respectivelyassociated with the two possible logical values of the at least onereceived bit.

According to one general feature of this other aspect, the processingunit comprises reception processing unit configured for implementing thereception method such as defined hereinbefore.

The processing unit may comprise both a transmission processing unit anda reception processing unit.

Such a processing unit may, for example, be a universal asynchronousreceiver/transmitter (UART).

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent uponexamining the detailed description of non-limiting embodiments and theirimplementations, and the appended drawings in which:

FIGS. 1 to 6 illustrate diagrammatically various embodiments of theinvention and their implementations.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In FIG. 1, the reference 1 represents a processing unit configured forexchanging information, for example bits, in an asynchronous manner,with another processing unit 2, over a serial link 3 here comprising twowires 30 for the transmission from the unit 1 to the unit 2 and twowires 31 for the transmission from the unit 2 to the unit 1.

Each serial link 30, 31 is capable of taking two different statesrespectively associated with the two possible logical values of thetransmitted bits. These two logical states result for example from thedifference in potential between the data wire and the ground wire.

The processing unit 1 comprises transmission processing unit 10 andreception processing unit 11. These two units are timed by a clocksignal CK1 coming from a generator 12. Similarly, the processing unit 2comprises transmission processing unit 20 and reception processing unit21 timed by a clock signal CK2 coming from a generator 22.

The transmission processing units (transmitters) 10, 20 are configuredfor implementing the transmission method, implementation examples ofwhich will be described with reference to FIGS. 2 and 4 to 6.

Similarly, the reception processing units (receivers) 11, 21 areconfigured for implementing the reception method of one of theimplementation examples which will be described with reference to FIGS.3 to 6.

For this purpose, the transmission processing units and the transmissionand reception processing units comprise elements with a conventionalstructure, such as for example shift registers and/or any other circuitscapable of generating edges over the transmission link depending on thelogical values of the bits to be transmitted, and/or any other circuitscapable of analyzing the characteristics of the received frame, forexample a microcontroller.

The processing units may for example be universal asynchronousreceivers/transmitters (UART).

Reference is now more particularly made to FIG. 2 in order to illustrateone embodiment of a method for transmission of a bit b over the seriallink.

It is assumed here that the logical value of the bit b is equal to 0 andthat the transmission start time t0 of the bit b corresponds to afalling edge TR0 starting from the steady state S1 (here high state) ofthe link.

The bit b has a bit time TB.

A first portion PTB1 of the bit time TB equal to αTB is then defined.

The transmission processing unit 10 of the processing unit 1 then placesthe serial link in one of its states depending on the logical value ofthe bit b for the whole of the first portion of bit time PTB1. In theexample described here, since the logical value of the bit b is 0, thelink is placed in its state S0.

Then, upon the expiration of the portion PTB1, a first additionaltransition TRA1 is generated over the link so as to place it in itsother state, in the present case the state S1, and this lasts up to theexpiration of the bit time TB in other words during the portion PRB1equal to (1-α)TB.

One embodiment of the reception processing of such a transmitted bit isillustrated in FIG. 3.

The reception processing unit 21 of the processing unit 2 knows thevalue of α.

The reception processing first of all comprises a detection of the timett0 of the start of reception of the bit b, here typically correspondingto the occurrence of a falling edge TR0 from the steady state S1 of thelink.

Then, the occurrence of a first transition TR1 between the two states ofthe link is detected, here between the state S0 and the state S1, afterthe time tt0.

This first transition takes place at the time tt1.

The reception processing unit of the processing unit then determines thefirst period D1 between the times tt0 and tt1.

The reception processing unit are from then on able to determine thatthe bit time TB of the received bit will be equal to D1/α and that thetime tt2 of the end of reception of the bit b will be separated from thetime tt1 of occurrence of the first transition TR1 by a first additionalperiod DA1 equal to a first predetermined portion β of the first periodD1, in this case defined by the formula (I) hereinbelow:

$\begin{matrix}{{{D\; A\; 1} = {\frac{\left( {1 - \alpha} \right)}{\alpha}D\; 1}}{{{with}\mspace{14mu} \beta} = {\frac{\left( {1 - \alpha} \right)}{\alpha}.}}} & (I)\end{matrix}$

It can therefore be seen that the use of an additional transition in thetransmission phase situated towards the end of the bit time in receptionallows the end of the reception of the bit to be anticipated and the bittime to be determined.

So as to reduce the risk of error in the detection of the bit time, itis preferable for the first additional transition TRA1 and, as aconsequence the first transition TR1, to be situated sufficiently farfrom half way through the bit time and the end of the bit time. Thoseskilled in the art will be able to choose α taking into account thesecriteria notably as a function of the value of the clock signals used.

This said, by way of example, α may be chosen within the range between0.6 and 0.9, for example equal to 0.75.

For this reason, β is in the range between 0.11 and 0.66, for exampleequal to 0.33.

In the examples in FIGS. 2 and 3, the time t0 of the start oftransmission of the bit b and the time tt0 of the start of reception ofthe bit b have been represented by a falling edge TR0. However, it goeswithout saying that, depending on the logical value of the bit and/or onthe steady state of the link, this transition TR0 could be a risingedge. Furthermore, in the case where several bits are transmittedsuccessively, if the bit b-1 preceding the bit b has a logical valueopposite to that of the bit b, then, for the bit b, there will be notransition TR0 following a transition TRA1 (in transmission) or atransition TR1 (in reception) generated during the bit time of the bitb-1. In this case, the time t0 of the start of transmission of the bit bwill be the time of the end of transmission of the bit b-1 and the timett0 of the start of reception of the bit b will be the time estimatedfor the end of reception of the bit b-1.

Reference is now more particularly made to FIG. 4 in order to illustrateone embodiment of a method of transmission applicable to a packet PQ ofbits.

In the example described here, the packet is a frame of the type UARTcomprising a first bit referenced “start”, which is a start bit followedby eight useful bits of data b0-b7 followed by a final bit referenced“stop”.

The transmission processing illustrated in FIG. 2 is applied here onlyfor the first bit start of the packet.

In fact, although it is theoretically possible for the bit time TB ofthe first bit start to be equal to the bit time TB0 of all the otherbits of the packet PQ, it is preferable, so as to minimize the risk oferror in the detection of the bit time in reception, for the bit time TBof the first bit start to be longer than the bit time TB0.

In practice, a bit time TB could be chosen to be equal to K times TB0,where K may be an integer or otherwise. K may, in particular, be amultiple N of the bit time TB0, where N may for example be chosen to beequal to 4.

The first additional transition TRA1 is, as indicated hereinbefore,generated upon the expiration of the portion PTB1 equal to αTB.

In reception, the time periods D1 and DA1 are determined as indicatedhereinbefore with reference to FIG. 3 and the bit time TB is then equalto D1/α as explained hereinbefore.

Once the bit time TB has been determined, the bit time TB0 can bereadily calculated by dividing the bit time TB by N.

Thus, the speed of transmission (“baud rate”) is determined.

Nevertheless, the bits b0 at stop could have various bit times butdeterminable from the bit time TB by a known relationship of thereceiver. Thus, each bit time TBi for the bits b0 at stop could be theproduct of TB with a coefficient C_(i), C_(i) being the coefficientassociated with the bit of rank i and also known by the receiver. αremains identical for all the bits.

In the example illustrated in FIG. 5, the packet PQ is a UART framecomprising here again a first bit start followed by eight data bitsb0-b7 followed by a final bit stop.

In this example, it is assumed that all the bits of the packet PQ havethe same bit time as TB.

In this embodiment, so as to reduce the risk of error in the detectionof the bit time in reception, and consequently in the detection of thespeed of transmission (baud rate), the transmission processing unit willgenerate an additional transition, not only during the bit time of thefirst bit start, but also during certain other bits of the packet, inthis case the bits b0 and b2.

More precisely, the first transmission processing, such as illustratedin FIG. 2, is applied for the first bit start leading to the generationof the first additional transition TRA1 at the end of the portion PTB1equal to αTB.

As illustrated in the upper part in FIG. 5, a second transmissionprocessing applied to the bit b0 comprises, starting from thetransmission start time of this bit b0 and up to the expiration of asecond period D20, which is this time counted starting from thetransmission start time of the first bit start, the placing of the linkin the state corresponding to the logical value of the bit b0, in thiscase the placing of the link in the state S0.

Accordingly, the link is placed in this state S0 during a second portionPTB2 of the bit time TB.

Although not indispensable, the duration of the portion PTB2 ispreferably equal to the duration of the portion PTB1, for the sake ofsimplification.

PTB2 is consequently equal to αTB.

The second period D20 is consequently equal to the sum of the bit timeTB associated with the start bit and of the second portion PTB2. D20 istherefore equal to (1+α)TB.

Upon the expiration of the second period D20, the transmissionprocessing unit generates a second additional transition TRA2 so as toplace the link in its state S1 up to the expiration of the bit time TBassociated with the bit b0.

The link is therefore placed in its state S1 during the portion of bittime PRB2 here equal to (1−α)TB.

In view of the generation of the two additional transitions TRA1 andTRA2, the bit time TB will be able to be calculated with sufficientprecision so as to be able to forego the generation of anotheradditional transition within the bit b1.

On the other hand, it is preferable to re-generate a third additionaltransition TRA3 during the bit b2.

This third additional transition TRA3 is generated upon the expirationof a third period D30 again counted starting from the time of the startof the first bit start, this duration D30 being equal to the sum of bittimes of preceding bits, in this case here three bit times TB, and ofthe portion PTB3 which is equal to αTB. D30 is therefore here equal to(3+α)TB.

In reception, as illustrated in the lower part in FIG. 5, the receptionprocessing unit applies, for the first bit start, the receptionprocessing such as that illustrated in FIG. 4, which allows not only theend of reception of the start bit to be determined but also a firstdetermination of the bit time TB to be obtained by applying the formulaTB=D1/α.

Then, the reception processing unit applies a second receptionprocessing for the bit b0.

This second reception processing comprises a detection of the occurrenceof a second transition TR2 over the link, here making it go from itsstate S0 to the state S1, after the reception time of the bit b0.

The reception processing unit then determines a second period D2separating the reception time of the first bit start and the time ofoccurrence of this second transition TR2.

The reception processing unit can then carries out a new determinationof the bit time TB which is equal to:

$\begin{matrix}{\left( \frac{1}{1 + \alpha} \right)D\; 2} & ({II})\end{matrix}$

This second determination of the bit time is more accurate than thefirst which becomes obsolete.

Furthermore, the reception end time of the second bit b0 will beseparated from the time of occurrence of the second transition TR2 by asecond additional period DA2 which is equal to a second predeterminedportion of the second period D1.

More precisely, the second additional period DA2 is equal to:

$\begin{matrix}{\left( \frac{1 - \alpha}{1 + \alpha} \right)D\; 2} & ({III})\end{matrix}$

The second reception processing is also applied for the fourth bit b2 ofthe packet PQ.

More precisely, the detection of the third transition TR3 allows thethird period D3 to be determined still counted starting from thereception start time of the first bit start.

This allows a new estimation of the bit time TB to be obtained which issupplied by the formula (IV) hereinbelow:

$\begin{matrix}{{T\; B} = {\left( \frac{1}{3 + \alpha} \right)D\; 3}} & \left( {I\; V} \right)\end{matrix}$

This third determination of the bit time is more accurate than thesecond which, in turn, becomes obsolete.

The reception end time of the bit b2 will then be obtained at the end ofthe expiration of the third additional period DA3 which is obtainedusing the formula (V) hereinbelow:

$\begin{matrix}{{D\; A\; 3} = {\left( \frac{1 - \alpha}{3 + \alpha} \right)D\; 3}} & (V)\end{matrix}$

Whereas in the embodiment in FIG. 5 which has just been described, thebit time TB was identical for all the bits, it could here again bedifferent for at least some of them, or even for all the bits, as longas these bit times are determinable by a known relationship of thetransmitter and of the receiver.

More precisely, it may for example be assumed that the bit time of eachbit of the packet is equal to GiTBB, where TBB is a constant and Gi acoefficient associated with the bit of rank i in the packet PQ. Thecoefficients Gi are known by the transmitter and by the receiver, and aremains identical for all the bits but could be different for each bitas long as it is known to the transmitter and receiver.

Thus, taking once again the example in FIG. 5, the period D1 allows thebit time TB1 of the start bit to be determined in reception by theformula TB1=D1/α.

This bit time TB1 is furthermore equal to G1TBB (where G1 is thecoefficient associated with the first bit start) which supplies a firstdetermination for TBB equal to D1/(αG1).

A new estimation of TBB can be obtained in reception with the transitionTR2 (second bit b0) and the formula TBB=D2/(G2(1+α)).

The bit time TB3 of the third bit b1 can then be determined by thereception processing unit via the formula TB3=G3TBB.

A new estimation of TBB can be obtained in reception with the transitionTR3 (fourth bit) and the formula TBB=D3/(G4(3+α)).

This latter estimation of TBB will be retained for the determination bythe reception processing unit of the bit times of the bits b3 at stopusing the formula G_(i)TBB.

The embodiment illustrated in FIG. 6 is more particularly adapted whenthe speed of transmission (baud rate) changes in the course of thecommunication, and even when it changes for each transmitted bit.

For this FIG. 6, the packet of bits PQ is still a packet of the UARTtype but this time the bit time of each of the bits may be different.

Accordingly, the application is advantageously provided at thetransmission, for each bit to be transmitted, the transmissionprocessing such as that described with reference to FIG. 2 and, inreception, for each bit received, the reception processing such as thatillustrated with reference to FIG. 3 by each time taking into account,as explained hereinbefore with reference to these figures, the manner inwhich the times of the start of transmission of a transmitted bit or ofthe start of reception of a received bit (absence or presence of atransition TR0) are determined.

In practice, the transmission processing unit will generate anadditional transition TRA1 during each transmission of a bit, at the endof the portion PTB1. In the example described, for the purposes ofsimplification, PTB1 is identical for all the bits since it was assumedthat the bit time was equal for all the bits. However, PTB1 could bedifferent for at least some of the bits owing to the difference in theassociated bit times. α can remain identical for all the bits but couldbe different for each bit as long as it is known to the transmitter andreceiver.

In reception, the reception processing unit will determine, for eachbit, the period D1, the additional period DA1, and will accordingly beable to determine the bit time associated with the bit in question.

The invention is not limited to the embodiments and theirimplementations which have just been described but encompasses all thevariants of the latter.

Thus, although in everything that has been described the transmission orreception start time of the first bit of a packet was determined by theoccurrence of a transition of the link starting from its steady state,it is perfectly possible for this transmission or reception start timeto be supplied by a signal external to the link.

What is claimed is:
 1. A method of transmitting a plurality of bits overa serial link that is capable of taking two different statesrespectively associated with two possible logical values of thetransmitted bits, the method comprising: starting from a transmissionstart time of a first bit and up to expiration of a first portion of abit time associated with the first bit, placing the link in one of thetwo different states depending on the logical value of the first bit;and upon expiration of the first portion of the bit time, generating afirst additional transition over the link so as to place the link in theother one of the two different states up to an expiration of the bittime.
 2. The method according to claim 1, wherein the first portion ofthe bit time is between 60% and 90% of the bit time.
 3. The methodaccording to claim 1, wherein the plurality of bits that form a packetand wherein transmission of a second bit begins at the expiration of thebit time associated with the first bit.
 4. The method according to claim3, wherein the bit time associated with the first bit of the packet islonger than a bit time associated with other bits of the packet and thebit times associated with the other bits of the packet may bedetermined, by a known relationship, from the bit time associated withthe first bit of the packet.
 5. The method according to claim 4, whereinthe other bits are transmitted by placing the link in only one state foreach bit, the state depending on the logical value of the bit beingtransmitted.
 6. The method according to claim 3, wherein the bit timeassociated with all the bits of the packet are identical, or differentfor some of them but determinable by a known relationship, the methodfurther comprising transmitting an other bit, the transmittingcomprising: starting from a transmission start time of the other bit andup to expiration of a period counted starting from the transmissionstart time of the first bit and equal to a sum of all of the bit timesassociated with earlier bits preceding the other bit and of a secondportion of the bit time associated with the other bit, placing the linkin one of the two different states depending on the logical value of theother bit; and upon the expiration of the period, generating a secondadditional transition over the link so as to place the link in the otherone of the two different states up to an expiration of the bit timeassociated with the other bit.
 7. The method according to claim 6,wherein the first portion and the second portion are the same.
 8. Themethod according to claim 6, wherein the first bit is the only earlierbit so that the period is equal to the bit time of the first bit.
 9. Themethod according to claim 3, wherein steps of placing the link in one ofthe two different states and generating an additional transition overthe link are performed for each bit of the packet, the transmission of acurrent bit, from a second bit up to a last bit, beginning at theexpiration of the bit time associated with the latest preceding bit. 10.A processing unit configured for a plurality of bits over a serial linkthat is capable of taking two different states respectively associatedwith two possible logical values of the transmitted bits, the processingunit comprising a transmission processing unit configured to implementthe method of claim
 1. 11. A method for receiving a bit carried over aserial link that is capable of taking two different states that areassociated with two possible logical values of the bit, the methodcomprising: detecting a first transition between the two states of thelink, the first transition occurring at a time after a reception starttime of the bit; and determining a first period separating the receptionstart time and the time of occurrence of the first transition, areception end time of the bit being separated from the time ofoccurrence of the first transition by a first additional period that isequal to a first predetermined portion of the first period.
 12. Themethod according to claim 11, wherein the first predetermined portion isbetween 10% and 70% of the first period.
 13. The method according toclaim 11, wherein the method comprises receiving a packet that comprisesa plurality of bits, the bit being a first bit of the plurality of bits,the method further comprising receiving a second bit beginning at anexpiration of the first additional period associated with the first bit.14. The method according to claim 13, wherein the bit time associatedwith the first bit of the received packet is longer than the bit timeassociated with other bits of the packet, the method further comprisingdetermining the bit time associated with the other bits from the bittime associated with the first bit of the received packet using a knownrelationship.
 15. The method according to claim 14, wherein the otherbits are received while in only one state for each bit, the statedepending on the logical value of the bit being transmitted.
 16. Themethod according to claim 13, wherein all the bits of the packet havethe same bit time, or have different bit times for some of them that aredeterminable by a known relationship, the method further comprisingreceiving an other bit of the packet, the receiving comprising:detecting an occurrence of a second transition between the two states ofthe link after a reception start time of the other bit; and determininga second period separating the reception start time of the first bit andthe time of occurrence of the second transition, the reception end timeof the other bit being separated from the time of occurrence of thesecond transition by a second additional period that is equal to asecond predetermined portion of the second period.
 17. The methodaccording to claim 16, wherein the first portion and the second portionare the same.
 18. The method according to claim 13, further comprisingreceiving each of the other bits of the packet after receiving the firstbit, wherein receiving each of the other bits includes detecting anadditional transition after a reception start time of that bit.
 19. Aprocessing unit configured to receive a packet of bits carried over aserial link, the processing unit comprising a reception processing unitconfigured to implement the method according to claim
 11. 20. A methodof communicating using a serial link, the method comprising:transmitting an initial start bit portion for a first time period, thestart bit having a start bit time; transmitting an additional start bitportion having a second time period, the additional start bit portionhaving an inverse value of the initial start bit portion and the startbit time being equal to the sum of the first time period and the secondtime period; and after the start bit time, transmitting a plurality ofuseful bits, each of the useful bits have a useful bit time, the startbit time being longer than the useful bit time and being an integermultiple of the useful bit time.
 21. The method according to claim 20,further comprising: receiving the initial start bit portion; receivingthe additional start bit portion; determining the start bit time basedon timing of receiving the initial start bit portion and the additionalstart bit portion; determining the useful bit time based on the startbit time and the integer multiple; and receiving the useful bits at abit rate based on the useful bit time.
 22. The method according to claim20, wherein each useful bit is transmitted at only a single level basedon a logical value of that useful bit.
 23. The method according to claim20, wherein the first time period is between 60% and 90% of the startbit time.